Oil country tubular goods such as tubing and casing used in the excavation of oil wells and gas wells are usually connected to each other by threaded joints for pipes. In the past, the depth of oil wells was typically 2,000-3,000 meters, but in deep oil wells such as those in recent offshore oil fields, the depth may reach 8,000-10,000 meters.
In their environment of use, threaded joints for connecting oil country tubular goods are subjected to loads such as axial tensile forces caused by the weight of the oil country tubular goods and the threaded joints for pipes themselves, the combination of internal and external pressures, and geothermal heat. Accordingly, they need to be able to maintain gastightness without undergoing damage even in such a severe environment.
A typical threaded joint for pipes used for connecting oil country tubular goods to each other has a pin-box structure. A pin is a joint component having a male thread formed on the end of an oil country tubular good, for example, and a box is a joint component having a female thread formed on the inner surface of a threaded connector (a coupling). In the case of a threaded joint referred to as a premium joint which has superior gastightness, unthreaded metal contact portions are formed at the tip of the male thread of the pin and at the base portion of the female thread of the box. The unthreaded metal contact portions can include a metal seal portion formed on a cylindrical surface of the pin or the box, and a torque shoulder which is nearly perpendicular to the axial direction of the threaded joint. When one end of an oil country tubular good is inserted into a threaded connector and the male thread of the pin and the female thread of the box are tightened, the unthreaded metal contact portions of the pin and the box are made to contact each other with a prescribed amount of interference so as to form a metal-to-metal seal and thereby provide gastightness.
During the process of lowering tubing or casing into an oil well, due to various problems, it is sometimes necessary to disconnect a joint which has been once connected, to lift it out of the oil well, to reconnect it, and then relower it. API (American Petroleum Institute) requires resistance to galling such that so-called galling or severe seizing does not occur and gastightness is maintained even if make-up (connection) and break-out (disconnection) are repeated ten times for a joint for tubing or three times for a joint for casing.
At the time of make-up, in order to increase galling resistance and gastightness, a viscous liquid lubricant which contains heavy metal powders and which is referred to as compound grease is applied to the contact surfaces (the threaded portions and the unthreaded metal contact portions of the pin and the box) of a threaded joint for pipes. Such compound grease is specified by API Bulletin 5A2. Compound grease also exhibits corrosion resistance by preventing the occurrence of rust on a contact surface to which it is applied.
It has been proposed to previously subject the contact surfaces of a threaded joint for pipes to various types of surface treatment such as nitriding, various types of plating including zinc plating and composite plating, and phosphate chemical conversion treatment to form one or more layers thereon in order to increase the retention of compound grease and improve sliding properties. However, as described below, the use of compound grease poses the threat of harmful effects on the environment and humans.
Compound grease contains large amounts of powders of heavy metals such as zinc, lead, and copper. When make-up of a threaded joint for pipes is carried out, grease which has been applied is washed off or overflows to the exterior surface, and there is the possibility of its producing harmful effects on the environment and especially on sea life, particularly due to harmful heavy metals such as lead. In addition, the process of applying compound grease worsens the work environment, and there is also a concern of harmful effects on humans.
In recent years, as a result of the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) for preventing ocean pollution in the Northeast Atlantic, strict restrictions concerning the global environment have been imposed increasingly, and in some regions, the use of compound grease is already being restricted. Accordingly, in order to avoid harmful effects on the environment and humans in the excavation of gas wells and oil wells, a demand has developed for threaded joints for pipes which can exhibit excellent galling resistance without using compound grease.
Another problem of compound grease is that it contains a large amount of a solid lubricant typified by graphite, so the coating is not transparent. A pin which has a threaded portion on the outer surface of a pipe is more easily damaged by problems during transport or during make-up than is a box having a threaded portion on the inner surface of a pipe, so the threaded portion of a pin is often inspected for damage prior to make-up operations. When a compound grease is applied to a pin, it is necessary to wash off the applied compound grease at the time of this inspection, and then it is necessary to reapply compound grease after inspection. As described above, this operation is harmful to the environment and troublesome. If a lubricating coating is transparent, a threaded portion can be inspected for damage without removing the coating, and inspection can be made much easier.
As a threaded joint which can be used for connection of oil country tubular goods without application of a compound grease, the present inventors previously proposed in WO 2006/104251 a threaded joint for pipes in which the contact surface of at least one of a pin and a box is coated with a two-layer coating comprising a viscous liquid or semi-solid lubricating coating and atop it a dry solid coating. The dry solid coating can be formed from a coating of a thermosetting resin such as an acrylic resin or a coating of a UV-curable resin. The viscous liquid or semi-solid lubricating coating is tacky and foreign matter easily adheres to it, but by forming a dry solid coating atop it, tackiness is eliminated. The dry solid coating is broken at the time of make-up of a threaded joint, so it does not interfere with the lubricating properties of the lubricating coating beneath it. This threaded joint for pipes has excellent lubricating properties and sufficient galling resistance, but it is necessary to form a two-layer structure of the lubricating coating and the dry solid coating, so costs become high. In addition, when the two-layer structure is broken at the time of make-up, flakes are formed, and its subsequent appearance is not very good. In addition, the coating has low transparency.
In WO 2007/042231, the present inventors disclosed a threaded joint for pipes in which a thin lubricating coating which is not tacky and which has solid lubricant particles dispersed in a solid matrix exhibiting plastic or viscoplastic rheological properties (flow behavior) is formed on the threaded portions of a pin and a box. The melting point of the matrix is preferably in the range of 80-320° C., and it is formed by spray coating in melt (hot melt spraying), flame coating using powders, or spray coating of an aqueous emulsion. The composition used in the hot melt method contains, for example, polyethylene as a thermoplastic polymer, a wax (such as carnauba wax) and a metal soap (such as zinc stearate) as lubricating components, and a calcium sulfonate as a corrosion inhibitor. This threaded joint for pipes has excellent lubricating properties and corrosion resistance. However, since the coating is not transparent, it is difficult to perform inspection for damage of the threads on the outer surface of the pin in preparation for situations in which galling suddenly occurs due to damage to the threads on the outer surface of the pin.
In WO 2006/75774, a threaded joint for pipes is disclosed in which the contact surface of at least one of a pin and a box is coated with a two-layer coating comprising a solid lubricating coating containing a lubricating powder and a binder, and atop it a solid corrosion protective coating which does not contain solid particles. This threaded joint for pipes has extremely high corrosion resistance, but the solid lubricating coating is a rigid solid coating having substantially no plastic or viscoplastic rheological properties. Therefore, even if the solid corrosion protective coating formed atop it is broken at the time of make-up of a threaded joint, it is difficult for the broken pieces to be embedded in the underlying solid lubricating coating, and its lubricating properties are relatively poor.